I make my regular buttercream recipe (1 package salted butter with a bit more than 2/3 cup shortening creamed, add vanilla, add 5 tablespoons whipping cream, 1 tablespoons light corn syrup and two pounds of powdered sugar - adjust for thickness etc.) then I microwave about a 1/4 cup chocolate chips with a spoonful of shortening, then add that in to my buttercream.

After the fats have been beaten to death, I add the sugar and cocoa, then keep mixing w/ Chocolate milk (plain milk w/choco syrup works too) until it's the proper consistency. It is SO GOOD! It crusts too.